Electrodynamic magnet system for microphones, loudspeakers, or similar devices



y 1935. R. VERMEULEN ET AL 2,006,847

ELECTRODYNAMIC MAGNET SYSTEM FOR MICROPHONES,

LOUD SPEAKERS OR SIMILAR DEVICES Or1g1nal Filed March 6, 1928 L Y %W% Emm /m N 8 W E VWT A N D. HUN 0 Patented July 2, 1935 UNITED STATESPATENT OFFICE ELECTRODYNAMIC MAGNET SYSTEM FOR,

MICROPHONES, LOUDSPEAKERS, OR SIM- ILAR DEVICES Corporation of America,

Delaware a corporation of Application March 6, 1928, Serial No. 259,445.

Renewed December 31, 1931.

lands March 25, 1927 11 Claims.

The invention relates to an electrodynamic magnet system formicrophones, loud-speakers or similar devices provided with wirewindings which are movably arranged in a magnetic field.

The invention has for its object to guide these windings during theirmovements in such a manner that at all frequencies and especially at thelower ones an efficiency is obtained which is as favorable as possible,

According to the invention, when the windings are moving in the magneticfield, they are kept centered by one or more layers of a substance whichpossesses a slightly elastic but strongly damping property. The slightlyelastic properties involve that the directing force which moves thewindings back into their middle-position, is small with the result thatthe natural frequency of the oscillating portion of the magnet system isvery low. If the radiated sound energy is graphically plotted forvarious frequencies, their curve is obtained which shows a peak at thenatural frequency of the oscillating portion of the magnet system. When,consequently, the magnet system oscillates at that frequency, theradiated sound energy will be considerably greater than at otherfrequencies, which is harmful to the sound reproduced. This disadvantagecan be eliminated by damping the movement of the oscillating portion ofthe magnet system and thereby smoothing out the peak.

The magnet system according to the present invention has the advantagethat the point of natural resonance comes to lie low while at the sametime a strong damping is obtained. Especially at frequencies rangingfrom 150 to 500 the efficiency of the system will be very favorable.

In a particular embodiment of the invention in which the magnet systemis provided with an annular coil which is movably arranged in an annularmagnetic field, this annular coil is kept centered by discs made of asubstance which possesses the above mentioned properties, said discsbeing secured to the poles or to parts of the magnet system which areimmovably connected to the said poles. The coil may be secured to thediscs but should preferably not be connected thereto.

In a suitable construction, a disc is further secured to the centralpole, another disc being secured to the annular pole.

The substance which has the abovementioned properties may consist oftextile fabric such as velvet or flannel, but it may also consist of asubstance such as felt.

The invention will be more clearly understood In The Netherby referringto the accompanying drawing in which Fig. 1 is a graph showing the valueof the radiated sound energy at various frequencies of the magnet systemaccording to the invention and Fig. .3 is a sectional view through thedriving mechanism of the invention.

In Figure 2, poles 2 and 3 of a permanent or electromagnet producetogether an annular magnetic field in which a coil 4 is movablyarranged. The coil 4 is wound on a small tube of cardboard which isdirectly secured to a cone 5. A felt layer 6 is secured to the pole 2 bymeans of a screw II, and an annular felt layer I is provided on the pole3 by means of an annular copper clamping piece 8 and screws 9. Thecardboard tube on which the coil 4 is wound, slides along the edges ofthe felt layers 6 and I and is thus kept centered in the annularmagnetic field. The felt possesses the property that after beingdeformed it has only a slight tendency to return into its originalposition while it exerts a strongly damping influence on the movement ofthe coil 4. When now this coil 4 is traversed by speech modulatedcurrent, it will experience a strong damping during its movements.Instead of felt, also several layers of textile fabric such as flannel,serge etc. may be utilized, all of them showing slightly elastic butstrongly damping properties.

In Figure 1 is indicated the value of the radiated sound energy atdifferent frequencies. The curve a shows the course of a magnet systemwith which the point of resonance lies very low. The curve b indicatesthe course of the magnet system of Figure 2. The point of resonance liesat a frequency of 200 while the oscil lations are strongly damped. Itappears from the figure that tones with frequencies ranging from 150 to400 cycles reproduce considerably better.

What we claim is:

1. A translating device comprising a magnet system, a vibratile member,a hollow cylindrical member secured to the vibratile member, a coilmounted upon the cylindrical member, a fabric disk fastened to part ofsaid magnet system and positioned to engage said cylindrical member,said fabric disk having slight elastic and strong damping properties.

2. A translating device comprising a magnet system, a vibratile member,a hollow cylindrical member secured to the vibratile member, a coilmounted upon the cylindrical member, and a fabric disk and an annularfabric member fastened to the magnet system and positioned to slidablyengage the hollow cylindrical member said disk being slightly elastic.

3. A translating device comprising a magnet system, a vibratile member,a hollow cylindrical member secured to the vibratile member, a coilmounted upon the cylindrical member, and a disk and an annular member ofyielding material fastened to the magnet system, said disk engaging theinside of the hollow cylindrical member and said annular member engagingthe outside of the cylindrical member.

4. An acoustic device of the electrodynamic type" comprising an innerpole piece, an outer pole piece spaced therefrom to form an air gap, 2.vibratile member, a driving member secured to said vibratile member andextending into said air gap, a washer of damping material secured tosaid inner pole piece and extending outwardly therefrom to engage theinner surface of said driving member, and an annular washer of dampingmaterial secured to said outer pole piece and extending inwardlytherefrom to engage the outer surface of said driving member.

5. An acoustic device of the electrodynamic type comprising acylindrical pole piece, an an nular pole piece spaced therefrom to forman annular air gap, a vibratile member, a cylindrical member securedthereto and extending into said annular air gap, a fabric disk securedto the top of said cylindrical pole piece and extending outwardlytherefrom beyond the circumference thereof to engage the inner surfaceof said cylindrical member, and an annular fabric member secured to saidannular pole piece and extending inwardly therefrom to engage the outersurface of said cylindrical member.

therefrom to engage said driving member, and a pair of annular memberssecured to said outer pole piece and extending inwardly therefrom, oneof said annular members engaging said driving member and the otherannular member engaging said central pole piece.

7. A sound converting device comprising a magnet system, a vibratilemember, a hollow cylindrical member secured to the vibratile 7 member, acoil mounted on the cylindrical memher and a fabric disk secured to partof said magnet system and positioned to engage said cylindrical member,said fabric disc being only slightly elastic whereby after beingdeformed it has only a slight tendency to return to its originalposition.

8. A sound converting device comprising a magnet system, a vibratilemember, a hollow cylindrical member secured to said vibratile member, acoil mounted on the cylindrical member, a fabric disc and an annularfabric member spaced apart and fastened to the magnet system andpositioned to slidably engage the hollow cylindrical member said discand fabric member being slightly elastic.

9. The device of the preceding claim in which the magnet comprises acentral pole and an annular pole and in which said fabric disc iscarried by said central pole and said annular fabric member by saidannular pole.

10. A sound converting device comprising the combination of a magnetsystem having an annular airgap and a pole piece located within saidgap, a vibratile diaphragm a substantially cylindrical coil formconnected to said diaphragm, a substantially cylindrical coil mounted onsaid coil form and at least two flexible members one being secured tosaid pole piece and engaging the interior surface of said coil form nearone end thereof and the other being secured to said magnet system andengaging the ex-

